Stress Wave Attenuation as An Indicator of Lumber Strength

Longitudinal stress waves were induced in Douglas-fir 1 by 4s selected to have various degrees of slope of grain. The amplitude of the stress waves was measured indirectly by a magnetic gage developed in this research to measure particle velocity. Equations are derived that show particle velocity to be directly related to the amplitude of the stress wave, which decays exponentially as a result of various mechanisms of internal friction. Qualitative results indicate that the major elements of one-dimensional wave theory could be observed using the magnetic gage. Quantitatively, the attenuation of the wave, along with other board parameters, can be used to improve the prediction of tensile strength. Preliminary results indicate that the magnetic gage also may have some practical application in evaluating finger joint quality.

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